Ionizing radiation, such as electron beams, gamma rays and X-rays, is widely used for the irradiation treatment of objects, including: the sterilization of medical, pharmaceutical, food and cosmetic products; the cross-linking of polymers and other industrial processes; the inactivation of leukocytes in transfusion blood supplies; the sterilization of insects for phytosanitary and research purposes; the attenuation of organism function for vaccine development, and many other purposes.
Broadly speaking, irradiators are classified as either self-contained irradiators or panoramic irradiators. In self-contained irradiators, the radiation source, radiation shielding, the objects to be treated, any systems for the movement of those objects, and sometimes the power supply, are all in one enclosure. X-ray versions are regulated by the U.S. Food and Drug Administration under the category “X-ray cabinet irradiator” (Title 21 CFR §1020.40). Panoramic irradiators are generally larger than the self-contained irradiators and use a material transport system to move the objects to be treated from an area where people may safely operate to a separately-shielded irradiation area receiving flux from the radiation source.
The radiation source used in either type of irradiator may include: gamma rays emitted by the decay of radioactive isotopes; electron beams produced by linear accelerators, electron tubes or other methods; or X-rays produced by the impact of high energy electrons upon a metal target, for example in an X-ray tube.
The predominant radiation sources for self-contained irradiators are radioactive isotopes and X-ray tubes. Radioactive isotope irradiators comprise: a sealed isotope source, most commonly Cesium-137 but in some cases Cobalt-60 [US NRC 2007]; a massive lead enclosure for this source, the lead commonly weighing over a ton; a vessel to hold the objects to be treated; and an internal transportation system to move this vessel from a cabinet section, where an operator may safely load and unload this vessel, to the lead-shielded radiation treatment section, all of these components being contained in one overall enclosure. The radioactive isotope constantly emits radiation due to natural radioactive decay. The isotope sources produce highly energetic gamma rays in the high keV (662 keV for Cs-137) to MeV (1.25 MeV for Co-60) range. The higher energy Co-60 sources require twice as much lead shielding as the Cs-137 sources, and also have a shorter half life, so Cs-137 has been the preferred source and was being used in most of the 1,341 isotope-based self-contained irradiators in the United States in 2007 [US NRC 2007].
About half of all isotope-based self-contained irradiators are used for blood irradiation and the remaining half for other purposes, including medical, scientific and agricultural research [US NRC 2007]. Blood is routinely irradiated at blood banks and hospitals to prevent the development of transfusion associated graft-versus-host disease (TA-GVHD) in immuno-suppressed patients. TA-GVHD is a usually fatal condition in which viable leukocytes in the transfused blood attack recipient organs and tissues. Irradiation renders the leukocytes unviable and is currently the only recommended method for GVHD prevention [BCSH Blood Transfusion Task Force]. Current guidance from the Food and Drug Administration (FDA) recommends a dose of 25 Gy delivered to the mid-plane of the blood container with no part of the blood container to receive less than 15 Gy. Most blood irradiators units in use today are self-contained, isotope-based systems using Cs-137. Many of the self-contained irradiators used in research also employ Cs-137 or Co-60. These typically deliver dose rates of 1-10 Gy/min to a cavity of 4 to 10 liters in size. These higher doses necessitate heavier shielding, so these units are larger and often weigh three or four tons. Even the smaller Cs-137 units take up valuable floor space at blood banks and hospitals and are cumbersome to operate.
The isotopes used in these irradiators could also be used in a radioactive dispersal device (“dirty bomb”) and have therefore become a major public security concern. Cs-137 and Co-60 account for nearly all (over 99 percent) of the sealed sources that pose the highest security risks in the United States [US NRC 2007]. Cs-137 is of particular concern since it is made in powder form and is therefore easily dispersible, because it has a relatively long half-life, and because it is present in major population centers. The primary use of Cs-137 is in self-contained irradiators for blood and research purposes. The National Research Council of the National Academy of Sciences' Committee on Radiation Source Use and Replacement identified Cs-137 as the top priority for the development of replacement technologies. Security concerns have added substantially to the acquisition and operating costs of irradiators using Cs-137.
Self-contained or cabinet irradiators using X-ray sources, such as those made by Faxitron X-Ray LLC, have found use in many applications, though generally not those now served by isotope irradiators. Most prior art X-ray irradiators use a single X-ray tube as the radiation flux source.
FIG. 1 shows the general architecture of prior art X-ray tubes. X-ray tubes are point sources of radiation, as shown in FIG. 1, wherein X-rays are generated by the impact of a high voltage electron beam 50 from a heated filament or other cathode 10 at a point (sometimes called the spot) on a metal anode 30, typically disposed at an angle relative to the cathode so as to allow X-ray flux 60 to exit one side of the vacuum tube enclosing the cathode and anode. This entire side may comprise the flux exit window of the tube, or a separate window 20 of a low Z material such as beryllium may be built into this side of the tube or housing for the tube. In tubes operating below cathode to anode voltages of 150 KV, less than 2% of the energy from the electrons is converted into X-rays, while the rest is dissipated as heat on the anode.
Though X-rays have long been known as a possible substitute radiation source for many of the uses of isotope-based self-contained irradiators, including blood irradiation [Janatpour 2005], several limitations of prior art X-ray irradiators have prevented their adoption. Irradiators using an X-ray tube will deliver an uneven dose to the irradiation target, for example a blood bag, since the X-rays will first impinge on one surface of the target and then be attenuated as they pass through the target material. X-rays from a single point on the anode will be emitted in all directions. Those which go back into the target will not be useful for irradiation, but will instead generate heat. With the X-ray target angled as shown in FIG. 1, even more of the X-rays are absorbed in the target than would be the case with a target disposed normal to the axis of the electron beam, a phenomenon known as the heel effect. Irradiation efficiency is further reduced by the fact that, of those X-rays directed away from the target, only those which impinge on the irradiation target surface will do useful work; the rest are absorbed by shielding structures. At the same time, the target surface area, to be useful in most irradiation applications, must be many times larger than the spot on the anode of an X-ray tube. As the intensity of the X-ray flux is inversely related to the square of the separation, the tube output has to be increased to meet the irradiation needs.
FIG. 2 shows the throw distance needed for prior art point sources used in irradiation. The cabinet and shielding must also be enlarged to accommodate the throw distance 200 shown in FIG. 2 that is required to cover a target area 400 with length and width 410. Furthermore, since all the flux needed for the application must come from one spot on the anode, there is a tremendous thermal load on this small area, which in turn necessitates the use of complex liquid cooling systems for higher flux applications.
Some recent inventions have taught the use of two or more X-ray tubes in a cabinet blood irradiator, such as U.S. Pat. Nos. 6,212,255 and 6,614,876. The X-ray tubes have been high-power models, with anode voltages of 160 kV, designed for applications such as computed tomography systems. While some aspects of an X-ray blood irradiator can be improved by using multiple tubes, rotating canisters are still needed to provide a uniform dose to the blood products, and the irradiator cabinet and shielding must be essentially twice as large to accommodate the flux throw distance from two tubes. Even with more than one tube, the use of a point source of X-rays still places a tremendous heat load on one spot, so externally-connected liquid cooling systems are still needed. In practice, these have proven to be cumbersome and unreliable, thereby limiting the adoption of X-ray systems for blood irradiation [Dodd, 2009].
More recently, a new type of specimen and blood irradiator consisting of a center-filament X-ray tube that irradiates 360 degrees around the tube and a cylindrical gold target has been described in U.S. Pat. No. 7,346,147. The electron source is a thermal cathode in the form of an elongated filament mounted along the axis of the cylindrically shaped transmissive type anode. Instead of a point source as is the case in most X-ray tubes, this invention is in the form of a line source. The electrons impinge on the interior surface of the anode and the X-rays generated penetrate the anode material and exit out of the exterior surface of the anode. The anode has to be made very thin (14 micron Au on 4 mil Al) in order to generate the forward directed X-rays. Flat panel versions of this kind of source using a transmissive anode are disclosed in U.S. Pat. Nos. 6,477,233 and 6,674,837. Two major limitations of this kind of source are the thermal loading capacity of the thin-film anode, and the thermal matching of the anode to the exit window of the source. Even with externally-connected liquid cooling systems, only limited amounts of X-ray power can be obtained from this kind of source. The X-ray irradiation apparatus taught by Avnery in U.S. Pat. Nos. 6,738,451, 7,133,493, and 7,324,630 also uses X-ray sources relying on a transmissive anode/X-ray target and thus having these same limitations.
Another X-ray source had been disclosed in U.S. Pat. No. 7,447,298 having a thermionic or cold cathode array inside a vacuum enclosure, which can direct e-beam current to a thin film X-ray target disposed on an exit window located above the cathode array with reference to the direction of the e-bam and X-ray fluxes, or, with a second cathode array, to a wide area anode located below the first cathode array, the second cathode arrays and the exit window with the thin-film anode. This source will have the heat dissipation limitations as discussed above for the thin-film X-ray target. X-rays produced by the lower, “reflective” anode will be attenuated first by the cathode arrays and their support structures, and then the thin-film X-ray target, resulting in an inefficient system. The second anode, while it can be thicker and have higher heat dissipation capacity than a thin-film anode, is inside the vacuum enclosure. The heat must therefore be transferred through the vacuum enclosure, which will limit the amount of X-ray flux that can be achieved with this source.